A primary barrier to xenotransplantation has been the essentially immediate recognition of carbohydrate epitopes present in the foreign tissue causing hyperacute xenograft rejection (HAR). The reaction begins immediately upon reperfusion, and once initiated destroys the foreign tissue within minutes to a few hours. The presence of HAR in some donor/recipient combinations while not others is postulated to be related to two primary factors, a) the binding of xenoreactive natural antibodies of the recipient to antigens or endothelial cells in the graft and b) the incompatibility of complement regulatory proteins in the transplant with the complement system of the recipient, allowing uncontrolled activation of complement. Greater than 80% of the complement-fixing natural antibodies in human serum recognize a single structure-Gal.alpha.1-3Gal. The synthesis of Gal.alpha.1-3Gal is catalyzed by the enzyme .alpha.1,3 galactosyl transferase.
No such recognition and destruction system exists for recognition of neoplastic cells and has remained the primary hurdle towards identification of an effective strategy for destruction of these cells. The ability to distinguish neoplastic from normal cells on the basis of proliferative behavior has proven limited, and has inspired a search for biochemical characteristics of neoplastic cells that are tumor specific rather than proliferation specific. Unfortunately current molecular genetic studies have failed to support the expectation that such characteristics are a consistent feature of neoplastic cells. Rather these studies suggest that the neoplastic state can be explained without postulating tumor specific functions, but merely the operation of normal proliferation-specific functions at abnormal levels, as a result of changes (sometimes minimal) in the structure of growth-regulatory genes or changes in their number or chromosomal environment. This conclusion suggests that a continued search for highly specific attributes of neoplastic cells cannot be relied upon for a general solution to the problems of cancer therapy. Major reductions in the lethality of cancer will require alternative approaches that do not depend on the natural occurrence of such attributes.
One alternative strategy entails the artificial creation of differences between normal and neoplastic cells through prophylactic use of gene insertion techniques. In other words, manufacturing biochemical differences which can be exploited to systematically and specifically target neoplastic cells for destruction. Gene insertion protocols are used to artificially manufacture biochemical differences in target tumor cells which are then exploited to selectively kill these cells. One system which has received much attention to date is the Herpes Simplex Virus Ganciclovir System.
Transformation of tumor cells with a gene encoding Herpes Simplex Virus thymidine kinase and subsequent treatment with anti-viral agents such as ganciclovir has been previously accomplished and has proven to be operable in vivo both in animals and in humans. See "Gene Therapy for the Treatment of Recurrent Pediatric Malignant Astrocytomas With In Vivo Tumor Transduction With Herpes Simplex Thymidine Kinase Gene/Ganciclovir System", Raffel, C et al., Human Gene Therapy 5 (7) p. 863-90, July 1994.
It is an object of this invention to introduce biochemical differences to tumor cells to allow for their selective killing via a complement mediated, xenoreactive natural antibody immune response.
It is a further object of this invention to provide compositions and methods for selectively killing tumor cells by complement destruction, through regulated expression of .alpha.1,3 galactosyl transferase.
It is a further object of the invention to provide compositions and methods for introduction of heterologous genes to tumor cells.
Other objects of the invention will become apparent from the description of the invention which follows.